The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

The surprising answer comes from parasites. Parasitic infestation can profoundly reduce the fertility and fecundity of a potential mate, so evolution places a very high premium on being able to detect whether your mate is infected. If the infestation occurred in early fetal life or infancy, one of the most obvious externally visible signs is a subtle loss of symmetry. Therefore, symmetry is a marker, or flag, for good health, which in turn is an indicator of desirability. This argument explains why your visual system finds symmetry appealing and asymmetry disturbing. It’s an odd thought that so many aspects of evolution—even our aesthetic preferences—are driven by the need to avoid parasites. (I once wrote a satirical essay that “gentlemen prefer blondes” for the same reason. It’s much easier to detect anemia and jaundice caused by parasites in a light-skinned blonde than in a swarthy brunette.)

Of course, this preference for symmetrical mates is largely unconscious. You are completely unaware that you are doing it. What a fitting bit of symmetry that the same evolutionary quirk in the great Mogul emperor Shah Jahan’s brain that caused him to select the perfectly symmetrical, parasite-free face of his beloved Mumtaz, also caused him to construct the exquisitely symmetrical Taj Mahal itself, a universal symbol of eternal love!

But we must now deal with the apparent exceptions. Why is a lack of symmetry appealing at times? Imagine you are arranging furniture, pictures, and other accessories in a room. You don’t need a professional designer to tell you that total symmetry won’t work (although within the room you can have islands of symmetry, such as a rectangular table with symmetrically placed chairs). On the contrary, you need carefully chosen asymmetry to create the most dramatic effects. The clue to resolving this paradox comes from the observation that the symmetry rule applies only to objects, not to large-scale scenes. This makes perfect evolutionary sense because a predator, a prey, a friend, or a mate is always an isolated, independent object.

Your preference for symmetrical objects and asymmetrical scenes is also reflected in the “what” and “how” (sometimes called “where”) streams in your brain’s visual processing stream. The “what” stream (one of two subpathways in the new pathway) flows from your primary visual areas toward your temporal lobes, and concerns itself with discrete objects and the spatial relationships of features within objects, such as the internal proportions of a face. The “how” stream flows from your primary visual area toward your parietal lobes and concerns itself more with your general surroundings and the relationships between objects (such as the distance between you, the gazelle you’re chasing, and the tree it’s about to dodge behind). It’s no surprise that a preference for symmetry is rooted in the “what” stream, where it is needed. So the detection and enjoyment of symmetry is based on object-centered algorithms in your brain, not scene-centered ones. Indeed, objects placed symmetrically in a room would look downright silly because, as we have seen, the brain dislikes coincidences it can’t explain.

Metaphor

The use of metaphor in language is well known, but it’s not widely appreciated that it’s also used extensively in visual art. In Figure 8.4 you see a sandstone sculpture from Kajuraho in Northern India, circa A.D. 1100. The sculpture depicts a voluptuous celestial nymph who arches her back to gaze upward as if aspiring to God or heaven. She probably occupied a niche at the base of a temple. Like most Indian nymphs she has a narrow waist weighed down heavily by big hips and breasts. The arch of the bough over her head closely follows the curvature of her arm (a postural example of a grouping principle called closure). Notice the plump, ripe mangoes dangling from the branch which, like the nymph herself, are a metaphor of the fertility and fecundity of nature. In addition, the plumpness of the mangoes provides a sort of visual echo of the plumpness and ripeness of her breasts. So there are multiple layers of metaphor and meaning in the sculpture, and the result is incredibly beautiful. It’s almost as though the multiple metaphors amplify each other, although why this internal resonance and harmony should be especially pleasing is anybody’s guess.